Not applicable.
The present invention relates to a method for manufacturing printing paper or paperboard with a grammage of 30-200 g/m2 in a paper or paperboard machine, comprising a wet section, a press section and a drying section, in which method a web, formed in the wet section, is pressed in a roll press with a doublefelted roll-press nip and, thereafter, in a shoe press with an extended single or double-felted shoe-press nip.
The invention also relates to a paper or paperboard machine for manufacturing printing paper or paperboard at high speed, which printing paper or paperboard has a grammage of 30-200 g/m2, comprising a wet section, a press section and a drying section, which press section includes a roll press, having a double-felted roll-press nip, and a shoe press, having an extended single or double-felted shoe-press nip.
U.S. Pat. No. 4,561,939 describes a paper machine with a press section, consisting of is a double-felted roll press and a double-felted shoe press. The roll press is of a conventional type having grooved rolls with rigid envelope suarfces. Such a configuration precludes high web speeds. After the press nip in the roll press, a suction shoe is arranged in the loop of the lower press felt, which suction shoe is intended to act so that the web accompanies the lower press felt. At high speeds, however, such a suction shoe cannot ensure such behaviour of the web. The suction shoe is an important element in the press section, according to this patent specification, which therefore does not disclose or suggest other suction devices to ensure the correct web behaviour at high speeds. Said specification employs a suction shoe after the second double-felted press nip as well, which therefore contributes to a further limitation of this known paper machine in respect of web speed. Said patent specification is limited to a double-felted shoe press for the second press nip and, thus, it does not disclose a transfer belt to replace one of the press felts to enable a secure web run and, thereby, higher speeds. Neither does it recognize the possibility of operating with a transfer belt at very high speeds and obtaining good dry-solids content also for mechanical pulp, which is used for manufacturing newsprint, LWC base paper and SC paper. The known paper machine fulrther lacks blowing boxes that generate partial vacuums to secure the firm attachment of the web to the press felt so as to enable high speeds, which result in strong air flows, which can easily detach the web from the press felt.
The object of the present invention is to provide an improved method and an improved paper or paperboard machine that enables the manufacture of printing paper or paperboard at very high speeds and that furtber enables high efficiency and a great increase in productivity.
The method, in accordance with the invention, is characterized in that
the web is pressed in a deflection-compensating roll press, having said double-felted roll-press nip and open press rolls,
the machine is operated at a web speed of at least 1,200 m/min.,
the web in said roll-press nip is subjected to a linear load ranging from 100 to 300 kN/m, preferably from 120 to 250 kN/m, and a specific pressure ranging from 5 to 15 MPa, preferably from 8 to 11 MPa,
the web in said shoe-press nip is subjected to a linear load ranging from 500 to 1,500 kN/m, preferably from 700 to 1,200 kN/m, and a specific pressure ranging from 4 to 13 MPa, preferably from 4 to 8 MPa,
to obtain a dewatered web with a dry-solids content of at least 35 percent, preferably at least 38 percent, after the roll-press nip and at least 45 percent after the shoe-press nip.
The paper or paperboard machine, in accordance with the invention, is characterized in that the roll press has open press rolls with deflection-compensating, rotatably journalled envelope surfaces and in that the machine is arranged to be operated at a web speed of at least 1,200 m/min., with a linear load in the roll-press nip ranging from 100 to 300 kN/m, preferably from 120 to 250 kN/m, and in the shoe-press nip ranging from 500 to 1,500 kN/m, preferably from 700 to 1,200 kN/m, and with a specific pressure in the roll-press nip ranging from 5 to 15 MPa, preferably from 8 to 11 MPa, and in the roll-press nip ranging from 4 to 13 MPa, preferably from 4 to 8 MPa, to obtain a dewatered web with a dry-solids content of at least 35 percent, preferably at least 38 percent, after the roll-press nip and at least 45 percent after the shoe-press nip.